It is known that 2-keto-L-gulonic acid can be produced by fermentation from L-sorbose and/or sorbitol.
FR 1 376 741 relates to a process for the preparation of 2-keto-L-gulonic acid, which is an intermediate in the production of L-ascorbic acid (=vitamin C).
It is demonstrated that the fermentative mixture can be used directly for esterification of 2-keto-L-gulonic acid into L-ascorbic acid.
EP 0 518 136 describes a method for preparing 2-keto-L-gulonic acid that comprises cultivating a mixed culture of micro-organism (A) which belongs to the genus Gluconobacter or Acetobacter, and a micro-organism (B) capable of producing 2-keto-L-gulonic acid from L-sorbose and both of said micro-organisms are co-existing in the medium during at least part of the entire cultivation period.
EP 0 278 447 relates to a fermentation process for producing 2-keto-L-gulonic acid by conversion of L-sorbose by means of mixed cultures of microorganisms comprising Gluconobacter oxydans and Bacillus megaterium. 
EP 0 972 843 relates to a continuous fermentation process for the manufacture of 2-keto-L-gulonic acid from sorbitol by fermentation with micro-organisms, in which process a nutrient medium containing sorbitol is incubated in a first fermentation vessel with a micro-organism capable of converting sorbitol to L-sorbose, and the resulting fermentation broth is transferred continuously to a second fermentation vessel where it is incubated with a micro-organism capable of converting L-sorbose to 2-keto-L-gulonic acid.
EP 0 213 591 describes a process for producing 2-keto-L-gulonic acid and vitamin C respectively and the method further describes that the isolation of 2-keto-L-gulonic acid may be effected by the formation of a salt or by using the difference in properties between the product and impurities such as solubility, adsorbability and distribution coefficient between two solvents. Although the use of an adsorbent such as ion exchange resins is described as being one of the most convenient processes for isolation of the product, the 2-keto-L-gulonic acid thus obtained is in general not pure.
EP 0 221 707 relates to a method for producing 2-keto-L-gulonic acid and describes a harvesting method wherein the culture broth is freed of cells by filtration, centrifugation or treatment with activated carbon and concentration of the solution. Solvent extraction, chromatography, precipitation or salting-out may be applied in a suitable combination and/or in repitition.
WO 01/09152 relates to a process for the purification of 2-keto-L-gulonic acid by continuous liquid chromatography using a weakly basic ion exchange resin. It mainly relates to recovering 2-keto-L-gulonic acid substantially water-free from aqueous solutions.
EP 0 359 645 relates to a process for obtaining pure 2-keto-L-gulonic acid from a fermentation broth containing the calcium salt of 2-keto-L-gulonic acid. Said process comprises separating the insolubles, concentrating the medium, precipitating calcium sulphate by adding concentrated sulphuric acid, treating with cation exchange resin, removing strong acid ions by anion exchanger, concentration and separating the 2-keto-L-gulonic acid by crystallisation.
EP 0 805 210 relates to a process wherein sodium 2-keto-L-gulonate is crystallised from the fermentation broth, the obtained crystals are pulverised and suspended in a lower alcohol containing a water-free acid and finally the salt of 2-keto-L-gulonate is converted into a lower alkyl ester of 2-keto-L-gulonic acid.
A further need exists to have a process for obtaining 2-keto-L-gulonic acid in high recovery yields and wherein the purification step is simple and easily reproducible.
The current invention provides such a process.